1. Field of the Invention
This invention relates to rock drilling equipment and more particularly drilling equipment utilised for drilling oil well bores and the like.
2. Description of the Prior Art
A problem which arises in operation of rock drilling equipment of the kind referred to is that occasionally the head of the operative end of the drill bit will break off from the remainder of the drill bit, due to the stresses arising in drilling and, in the absence of measures taken to prevent this, the broken off head of the drill will remain at the bottom of the bore being drilled and thus possibly many hundreds of feet below the surface. If the remainder of the drill string, with the remainder of the bit, is then withdrawn from the bore and the broken drill bit replaced, drilling of the bore cannot be resumed until the broken-off drill bit head is recovered, as otherwise the new drill bit would be bearing upon the broken drill bit head within the bore and not upon the rock. would present an inpenetratable barrier to the new drill bit and would merely result in the new drill bit in turn becoming hopelessly damaged. Various schemes have been proposed in the past for retrieving broken drill bit parts from bores or for ensuring that, should the operative part or head of a drill bit break off from the remainder, that drill bit head will nevertheless be retained on the end of the drill string allowing it to be retrieved from the bore with the drill string when the string is lifted from the bore. It is an object of the present invention to provide an improved arrangement of the latter sort.
According to the present invention, there is provided a rock drilling apparatus including a drill string terminating in a drill chuck and a drill bit detachably fitted into the drill chuck, the drill bit having a shank received in the drill chuck, the drill chuck and the drill bit shank having complementary non circular cross-section portions allowing rotational movement about the drill axis to be imparted to the drill bit via the drill chuck whilst allowing limited longitudinal movement of the drill chuck relative to the drill bit, the drill bit shank and the chuck having complementary formations above a first location along the drill bit, for limiting downward movement of the drill bit in the drill chuck, the drill bit having a drill head at its lower end which is of greater diameter than said shank, said head having a retaining formation above said lower end and the below said first location which is of greater diameter than the drill bit is at said first location, which retaining formulation co-operates with complementary retaining means on said chuck so as to prevent the complete detachment of said drill bit head from the chuck in the event of a fracture of the drill bit at said first location, and wherein said retaining formation on the drill bit head and the complementary retaining formation on the chuck take the form of respective parts of a bayonet connection arrangement which is substantially unstressed in normal operation of the drill, the arrangement being such that, in normal operation, the weight of the drill string, and the weight of the drill bit during lifting of the drill string, are supported by said complementary formations, above said first location which likewise receive the stresses imparted to the drill bit to cause a rotational movement of the drill bit.
Preferably the bit head retaining arrangement includes means associated with the arrangement for preventing rotation of the drill bit head relative to the chuck assembly without disassembly of the chuck assembly.
An embodiment of the invention is described below by way of example with reference to the accompanying drawings in which: